The demand for clean, secure, and renewable energy has stimulated great interest in fuel cells. A fuel cell is a device that converts chemical energy from a fuel into electricity through electrochemical reactions involving oxygen or another oxidizing agent. Fuel cells are different from batteries in that they require a constant source of fuel and oxygen to run, but they can produce electricity continually, so long as these inputs are supplied.
There are many types of fuel cells, but they all consist of an anode (negative side), a cathode (positive side) and an electrolyte that allows charges to move between the two sides of the fuel cell. Electrons are drawn from the anode to the cathode through an external circuit, producing direct current electricity. The main difference between the various types of fuel cells is the electrolyte. Thus, fuel cells are classified by the type of electrolyte they use. There are many different types of fuel cells, including molten carbonate fuel cells (MCFC), phosphoric acid fuel cells (PAFC), alkaline fuel cells (AFC), polymer electrolyte membrane fuel cells (PEMFC), and many more.
Solid Oxide Fuel Cells (SOFCs) are a particular type of fuel cell that uses a solid oxide or ceramic as the electrolyte of a cell. SOFCs are also known as high temperature fuel cells because the solid phase electrolytes usually do not show acceptable conductivity until they reach a high temperature of 800-1000° C. The solid oxide fuel cell is generally made up of three ceramic layers (hence the name): a porous cathode, a porous anode, and an electrolyte. SOFCs can have a fourth layer, called an interconnect layer, used to stack multiple fuel cells together. Hundreds of the single cells are typically connected in series or parallel to form what most people refer to as an “SOFC stack.” A basic SOFC is shown in FIG. 1, which illustrates a single cell on the left and a stack of cells on the right.
One of the important benefits of SOFCs is that SOFC systems can run on fuels other than pure hydrogen gas. This is because the high operating temperatures allow SOFCs to internally reform light hydrocarbons such as methane (natural gas), propane and butane to the H2 and CO needed for the fuel cell reactions. Heavier hydrocarbons including gasoline, diesel, jet fuel and biofuels can also serve as fuels in a SOFC system, but an upstream external reformer is usually required.
Among the many types of fuel cells, the SOFCs represent the cleanest, most efficient, and versatile energy conversion system, offering the prospect of efficient and cost effective utilization of hydrocarbon fuels, coal gas, biomass, and other renewable fuels. However, SOFCs must be economically competitive to be commercially viable and high operating temperatures and expensive materials contribute to cost.
One approach to cost reduction is to drastically reduce the operating temperature from high to intermediate temperatures, usually about 400-700° C., thereby allowing the use of much less expensive materials in the components and improving system longevity. Unfortunately, lowering the operating temperature also lowers the fuel cell performance, as the electrolyte and electrode materials become less conductive and less catalytically active.
Long-term performance of SOFCs also degrades due to poisoning of the cathode by chromium from interconnect layers, deactivation of the conventional anode by carbon deposition, and poisoning by contaminants (e.g., sulfur) in the fuel gas.
Oxygen ion conductors are the conventional conductors for electrolyte use in SOFC (e.g., FIG. 1). However, today both proton and mixed ion conductors are available for SOFC use. The reaction chemistry and examples of oxygen-ion conductors and proton conductors are shown in Table 1:
TABLE 1Oxygen ion and proton conductorsType of conductorOxygen ionProtonAnode reaction2H2 + 2O2− → 2H2O + 4e−/H2 → 2H+ + 2e−CO + O2−→ CO2 + 2e−Cathode reactionO2 + 4e− → 2O2−2H+ + 2e− + 1/2 O2 → H2OOverall reaction2H2 + O2 → 2H2O/2H2 + O2 → 2H2O2CO + O2→ 2CO2AdvantagesH2O, CO2 and high temperatures atNo fuel dilutionanode (fuel side) facilitates reformingIntermediate operating temperatureof hydrocarbon fuels to H2 and CO, orthrough water-gas shift reaction toproduce H2 from CODisadvantagesHigh operating temperature degradesReforming at anode (fuel side) lostsystem components and adds to costH2O formed at anode dilutes fuel incase of pure H2 usedExamplesYttria-stabilized zirconia (YSZ)Yttria-doped BaZrO3 (BYZ)Samarium doped ceria (SDC)Calcium-doped lanthanum niobateGadolinium doped ceria (GDC)(LCaNb)Scandia stabilized zirconia (ScSZ)Samarium-doped BaCeO3 (BCS)Strontium and magnesium dopedBarium-zirconium-cerium-yttriumlanthanum gallate (LSGM)(BZCY)Barium-zirconium-cerium-yttrium-ytterbium (BZCYYb)
As is well known, the advantages of SOFCs based on oxygen ion conductors include the formation of H2O and CO2 on the fuel side of the cell, which facilitates the use of carbon containing fuels through steam (H2O) and dry (CO2) reforming and water-gas shift reactions. However, the reaction products (e.g. H2O) also dilute the fuel. Further, the high operating temperatures of most oxygen ion conductors adds to cost and degrades system components.
The most important advantages of proton conducting solid oxide fuel cells (PC-SOFCs) compared to conventional oxygen type SOFCs rest on their fundamentally different working principles. In PC-SOFCs, protons migrate through the electrolyte from the anode to the cathode, and react with oxygen to form water. Because water forms at the cathode (not at the anode as for a conventional SOFC), dilution of the fuel at the anode is avoided and the anode environment remains reducing and independent of the fuel cell load. Absence of fuel dilution and the possibility to operate under higher loads without jeopardizing the anode yield potentially higher efficiency and maximum power for PC-SOFCs. However, direct utilization of carbon-containing fuels is no longer possible with proton-conducting electrolytes because the reforming reactions at the anode are no longer possible.
A third option is to tailor the proton and oxygen ion transference number of the mixed ion conductor, allowing CO2 to form on the fuel side while allowing most of the H2O to form on the air side. The class of mixed proton and oxygen ion conductors holds great potential for a new generation of low temperature SOFCs. However, to date, the ideal mixed ionic conductor has not been found.
Thus, in order to make SOFCs fully fuel-flexible and cost-effective power systems, the issues of anode tolerance to coking and sulfur poisoning, slow ionic conduction in the electrolyte and sluggish kinetics at the cathode need to be addressed. In a broader scientific context, the chemical and electrochemical mechanisms that lead to both of these issues and the phenomena that could prevent them should be investigated in order to best optimize the materials and microstructure of SOFCs for excellent performance and stability.
One attempt to make an ideal SOFC is in U.S. Pat. No. 7,749,626, which discloses an electrical current generating system comprising two stacks of SOFCs so that the gas exhaust and waste heat discharged from the first stack of SOFCs can be recycled and reused by the second stack of SOFCs and/or the reformer that reforms the fuel to hydrogen gas and/or carbon monoxide. The second stack can be a polymer electrolyte fuel cell stack or a phosphoric acid fuel cell stack. However, this patent focuses on the monitoring the exhaust content and power output, and modulating the gas content to be fed to the second stack of SOFC so as to control the power output. The patent teaches only the use of SOFC-O (e.g., col. 14, lines 39-41, “Oxygen ions produced at the cathode 56 move in the solid oxide electrolyte 55 of stabilized zirconia (YSZ or the like) and reach the anode 54”), without any teaching of the use of SOFC-H. In addition, U.S. Pat. No. 7,749,626 requires an additional reformer to reform the exhaust gas and additional fuel before they are supplied to the second stack of SOFC, which will increase operational cost to the SOFCs. Further, combining two SOFC-O cells together is not the most thermodynamically efficient way for SOFC operation because the second stack of SOFC-O cells also requires high operating temperature to achieve better ionic conductivity and overall efficiency.
Therefore, what is needed in the art are better SOFC fuel cells that can use methane as a fuel source and that are cost effective, efficient and long lasting.